Recalcitrant plastics accumulate in the environment at a rate of 25 million tons per year. The fate of these organic polymers in the environment and the time required for their total mineralization to CO.sub.2 have yet to be fully understood. There is a growing interest in the development of degradable plastics in landfills and composts. One of the most commonly suggested uses for starch-based degradable plastics is for composting of lawn, garden, and shrub litter, which could reduce the volume of material entering the landfills up to 20%.
The degradable plastic must still retain all of the physical properties expected by the consumer and, then, when placed in the appropriate environment, degrade more rapidly than conventional disposable plastics. To enhance the degradation of the polyethylene, chemical or photo-initiators or both are added to the degradable plastic films. For polyethylene films containing photo- and pro-oxidants, the primary initiators of oxidation are light and temperature, respectively. Both the pro-oxidant and the photo-oxidant produce free radicals on the long polyethylene chain, causing the material to lose some of its physical properties, to become oxidized, and possibly to become more accessible to microbial biodegradation. For further details with regard to conventional polyethylene or other alpha-olefin polymers containing photo- or pro-oxidants to enhance their degradation, see Chanda, M., and S. K. Roy. 1986, Plastic Technology Handbook, Marcel Dekker, Inc., New York; Cornell, J., A. M. Kaplan, and M. R. Rogers, 1984, Biodegradability of Photooxidized Polyalkylenes, J. Appl. Polym. Sci. 29:2581-597. Photo and/or pro-oxidant containing polyethylene is well known and need not be described in detail here. It is common for such polymers to contain amounts of starch polymer to enhance degradation. The amount of starch polymer will range from about 3% to about 12%, with about 6% to about 9% being typical.
While these starch containing polymers, especially those containing either photo- or pro-oxidants as well, are better than the nonstarch polymer containing alpha-olefin polymers from the standpoint of biodegradability, they still are less than totally satisfactory in achievement of the goal of complete biodegradability. Put another way, the polymers tend to remain environmentally persistent. There is, therefore, a continuing need to enhance degradation of alpha-olefin polymers.
There is a particular need to enhance the degradation of high molecular weight polyethylene which is so common in our environment. High molecular weight polyethylene, that is polyethylene having a molecular weight within in the range from about 50,000 to 90,000 or even higher, is particularly environmentally persistent. These materials are commonly produced with linear-low density polyethylene. In sum, the past efforts have not solved the environmental persistence of long chain high molecular weight polyethylene and the potential pollution therefrom.
This invention has as its primary objective, a method and means for enhancing the biodegradability of alpha-olefin polymers, particularly linear low-density polyethylene to CO.sub.2. As a result of this invention, the degradable polyethylene can be placed in landfills and composts.
The method of accomplishing the primary objective of the present invention is premised upon the discovery that in the presence of lignocellulose-degrading microorganisms starch containing degradable plastics, in pure culture, resulted in biodegradation of those plastics. Particulars of the method and of achieving the objectives of the invention are disclosed hereinafter.